In the field of semiconductor devices, thin nanosheets are generally required for good electrostatic control for devices having short channel lengths. Variations of thickness in a nanosheet due to, for example, imperfect selective etching, can result in degradation of device performance and an inferior device. For example, portions of a nanosheet that are thicker than a desired thickness may contribute to a loss of electrostatic control and a resulting degradation of performance, such as higher required threshold voltage Vt at a given off current Ioff. Similarly, portions of the nanosheet that are thinner than a desired thickness may result in a lower effective value of the current (Ieff) than desired. Thus, uniform thickness of nanosheets is desirable.
Furthermore, if electrostatics is sufficient, it may be desirable for nanosheets to have widths as wide as possible, ideally spanning an entire available portion of the cell height. Introducing breaks in nanosheets in order to make the width of the individual stacks smaller and more amenable to processing, for example, undercut of sacrificial layers without affecting thickness of the conduction layers in the nanosheet field effect transistor (FET) stack, may result in increased contribution to parasitic capacitance. The space between the sheets adds a parasitic gate to drain capacitance Cgd component, which may be detrimental to circuit performance. Thus, in addition to uniform, thin nanosheets, nanosheets as-wide-as-possible compatible with electrostatics, and precisely controlled are desired.
Conventional nanosheet fabrication includes epitaxial growth of the desired layers. For example, silicon germanium (SiGe) or silicon (Si) nanosheets are epitaxially formed yielding substantially defect-free Si/SiGe/Si/SiGe/Si . . . stacks, as long as the thickness of each layer is grown below a critical thickness hc. The critical thickness hc may vary from one device to another. Thus, the conventional stack of alternating Si and SiGe layers may provide defect-free stacks, but the critical thickness hc limitation put a ceiling of the maximum thickness of the nanosheets.
Furthermore, the selectivity ratio (SR) of Si versus SiGe, either from the wet etch using Tetramethylammonium hydroxide (TMAH) or a dry technique using hydrochloric acid (HCl), may be quite poor, for example, less than 5 and less than 30, respectively, due to the chemical similarity of silicon and germanium. This suggests that Si/SiGe/Si/SiGe . . . /Si stacks are unlikely to provide tight control of the shape of the nanosheets, thus, resulting in a deviation from the target performance and/or electrostatic control.